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Background: We have encountered instances where young subjects with idiopathic spontaneous pneumothorax have been needlessly referred for investigation of pulmonary hypertension because surgical pathologists have misinterpreted the significance of medial hypertrophy and intimal fibrosis of muscular pulmonary arteries in lung resection specimens., Methods: We reviewed 20 cases of idiopathic spontaneous pneumothorax and determined the prevalence and severity of medial and intimal lesions in the pulmonary arteries and pulmonary veins. We correlated the vascular changes with inflammation and fibrosis in the lung., Results: Pulmonary artery medial hypertrophy was seen in 15% of cases, pulmonary artery intimal fibrosis in 90% of cases, and pulmonary vein intimal fibrosis in 80% of cases. In 95% of cases, the lung showed some fibrosis and chronic inflammation. There was a significant positive correlation between pulmonary artery medial thickness and lung fibrosis and inflammation scores., Conclusions: Pulmonary artery medial hypertrophy and intimal fibrosis of pulmonary arteries and pulmonary veins are commonly seen in resected lung tissue from patients with idiopathic spontaneous pneumothorax. The vascular lesions are probably secondary to chronic inflammation and fibrosis in the adjacent lung. They are not clinically significant and do not represent hypertensive pulmonary vascular disease.

Collateral ventilation can participate in ventilation-perfusion regulation by shifting normoxic gas into hypoxic lung regions. In species lacking collateral pathways, such as cattle and swine, ventilation-perfusion balance must rely heavily on hypoxic vasoconstriction, which may explain why their muscular pulmonary arteries are much thicker than those of other animal species. The presence of these unusually muscular vessels in turn may account for the vigorous pressor response to acute hypoxia in these species. The only other species known to lack collateral ventilation is the coati. To determine whether coatis fit the pulmonary circulatory pattern of cattle and swine, we measured pulmonary arterial wall dimensions and pulmonary vascular responsiveness to acute airway hypoxia in 11 adult coatis. Hypoxia caused impressive pulmonary arterial hypertension [normoxia = 17 +/- 1 (SE) Torr, hypoxia = 40 +/- 2 Torr, cardiac output unchanged]. The medial thickness of muscular pulmonary arteries (50-300 microns) was 17.1 +/- 1.8% (SD) of external diameter, a thickness unprecedented in normotensive adult mammals. We conclude that coatis fit the pattern of other species lacking collateral ventilation, since they have thick-walled pulmonary arteries and a vigorous pressor response to hypoxia.

Qualitative and quantitative studies were performed on pulmonary blood vessels in lung tissue obtained by biopsy, pneumonectomy, or autopsy from 58 patients in the Registry of Primary Pulmonary Hypertension sponsored by the Heart, Lung, and Blood Institute of the National Institutes of Health. In 49 patients (84%), the hypertensive vascular disease involved predominantly or exclusively muscular pulmonary arteries and arterioles. In each of these 49 patients, pulmonary artery medial hypertrophy was observed, and in 48 patients, it was also associated with intimal or luminal lesions. On the basis of the predominant histopathologic features, 25 of the 48 patients were classified as having pulmonary arteriopathy with plexiform lesions characterized by a combination of concentric laminar intimal fibrosis, eccentric intimal fibrosis, and plexiform lesions; in nine of these 25, recanalized thrombi were also present. Pulmonary arteriopathy with thrombotic lesions, defined by the presence of both eccentric intimal fibrosis and recanalized thrombi but without plexiform lesions, was observed in 19 patients. Intimal fibrosis, either concentric or eccentric, without plexiform or thrombotic lesions was found in four patients. Among the remaining nine patients in the Registry, pulmonary veno-occlusive disease was present in seven and chronic pulmonary venous hypertension in one. Pulmonary blood vessels were microscopically normal in a lung biopsy specimen from another patient. In general, patients with plexiform lesions and those with veno-occlusive disease had a much poorer prognosis than patients with thrombotic lesions. The present study shows the existence of several distinct histopathologic patterns of pulmonary vascular disease in individuals with primary pulmonary hypertension diagnosed by standardized clinical and laboratory criteria.